1. Technical Field
The present invention relates to a discharge lamp lighting device for lighting a discharge lamp by high-frequency power generated by an inverter, and particularly to a discharge lamp lighting device having a simple configuration for performing dim control for a discharge lamp stably.
2. Background Art
Here, inspection will be made upon a conventional discharge lamp lighting device. FIG. 12 is a circuit diagram of a conventional discharge lamp lighting device, and FIG. 13 is a high-frequency voltage waveform diagram. In FIG. 12, the reference symbol E designates a DC power supply; IV, an inverter for inverting a DC voltage into a high-frequency voltage; LA, a discharge lamp having preheating electrodes F1 and F2; T, a ballast choke for limiting a discharge lamp current of the discharge lamp LA; C5, a coupling capacitor connected between the ballast choke T and the preheating electrode F2; C6, a starting capacitor connected between both the terminals of the discharge lamp LA; and FB, a feedback circuit for controlling the oscillation frequency so as to keep the output in a set value.
Next, the circuit configuration of the inverter IV will be described. Q2 and Q3 designate MOS FETs which are switching elements. In the MOS FET Q2, the drain is connected to the DC power supply, the source is connected to the drain of the MOS FET Q3, and the gate is connected to a pin 2 of an IV control integrated circuit IC2 which will be described later. In the MOS FET Q3, the source is connected to the DC power supply E through a detection resistor R6, and the gate is connected to a pin 4 of the IV control integrated circuit IC2.
The reference symbol R1 designates a starting resistor connected to the DC power supply E; C3, a control power capacitor connected between the starting resistor R1 and the earth; DZ, a voltage regulating diode for stabilizing the voltage of the control capacitor C3; IC2, an IV control integrated circuit for controlling the inverter IV. In the IV control integrated circuit IC2, the reference numeral 1 designates a power supply input terminal connected to a junction point between the control power capacitor C3 and the starting resistor R1; 2 and 4, voltage output terminals from which driving voltages for the MOS FET Q2 and Q3 are outputted; 3, a reference voltage output terminal; 6, a current output terminal (main oscillation resistor connection terminal) from which a current for determining resonance frequency is outputted; and 7, a current input/output terminal for charging/discharging a capacitor C4.
The description will be made below about the configuration of the feedback circuit FB. The feedback circuit FB is constituted by: resistors R2 and R3 for determining a current flowing out of the voltage output terminal 6; a capacitor C4 connected to the current input/output terminal 7; the source resistor or detection resistor R6 for detecting a high-frequency voltage flowing into the discharge lamp LA; an integrating circuit IN constituted by a resistor R5 and a capacitor C8 for averaging the high-frequency voltage detected by the detection resistor R6; and an error amplifier EA. The error amplifier EA is constituted by an operational amplifier IC3 and voltage dividing resistors R9 and R10 which are connected in series between the negative electrode of the power supply E and the junction point between the resistor R1 and the capacitor C3. The operational amplifier circuit IC3 is arranged such that the non-inverted input terminal thereof is connected to a reference voltage from the junction point between the resistors R9 and R10, while the inverted input terminal thereof is connected to a series connection of a capacitor 2, a diode D5 and the resistor R3 connected to the current output terminal 6 of the IV control integrated circuit IC2, thereby making the output voltage of the integrating circuit IN equal to the reference voltage.
Next, description will be made about the operation of the conventional discharge lamp lighting device with reference to FIGS. 12 and 13. FIG. 13 is a waveform diagram of a high-frequency voltage flowing into the discharge lamp LA when the discharge lamp is lighted.
First, the operation of the inverter circuit IV will be described. When the DC power supply E is turned on, a driving current flows in a closed loop of the power supply E the starting resistor R1, the control power capacitor C3, and to the power supply E, so that the control power capacitor C3 is charged. The voltage of the control power capacitor C3 is applied to the pin 1 of the IV control integrated circuit IC2. When the voltage of the control power capacitor C3 increases and reaches the working voltage of the IV control integrated circuit IC2, the IV control integrated circuit IC2 begins oscillation. With this oscillation, a high-frequency voltage is applied to the gate of the MOS FET Q2 of the half-bridge inverter circuit IV from the pin 2 of the IV control integrated circuit IC2, so that the MOS FET Q2 is turned ON. In addition, a low-frequency voltage is applied to the MOS FET Q3 from the pin 4 of the IV control integrated circuit IC2. Accordingly, the MOS FET Q2 and the MOS FET Q3 perform on-off operation alternately, so that the inverter circuit IV oscillates with a high frequency.
Consequently, a current flows alternately, in a closed loop, from the power supply E, to the preheating electrode F1, to the starting capacitor C6, to the preheating electrode F2, to the coupling capacitor C5, to the ballast choke T, to the MOS FET Q3, to the detection resistor R6, to the power supply E when the MOS FET Q3 is on, while, in the closed loop, from the coupling capacitor C5, to the preheating electrode F2, to the starting capacitor C6, to the preheating electrode F1, to the MOS FET Q2, to the ballast choke T, and to the coupling capacitor C5 when the MOS FET Q2 is on, so that a high-frequency current flows in a series circuit of the ballast choke T, the coupling capacitor C5, the preheating electrode F2, the starting capacitor C6, and the preheating electrode F1.
At this time, there is a relation that the capacitance value of the coupling capacitor C5 is sufficiently larger than the capacitance value of the starting capacitor C6. Accordingly, a high-frequency high voltage is generated in the starting capacitor C6 by the LC series resonance of the ballast choke T and the starting capacitor C6. This high-frequency high voltage is applied to the discharge lamp LA, so that the discharge lamp LA is lighted.
On the other hand, at this time, the high-frequency voltage generated in the detection resistor R6 is averaged by the integrating circuit IN of the feedback circuit FB, and this DC voltage is inputted into the inverted input terminal of the operational amplifier IC3 of the error amplifier EA. Then, the oscillation frequency of the IV control integrated circuit IC2 is determined by the capacitance value of the capacitor C4 and the value of a current flowing out to the resistors R2 and R3 from the current output terminal 6 of the IV control integrated circuit IC2. The larger this current value is, the higher the oscillation frequency becomes.
The current flowing into the resistor R3 from the current output terminal 6 changes in accordance with a change of the output voltage of the operational amplifier IC3, so that the oscillation frequency of the IV control integrated circuit IC2 is controlled.
Therefore, the oscillation frequency of the IV control integrated circuit IC2 is controlled by controlling the output voltage of the operational amplifier IC3 so that the output voltage of the integrating circuit IN is made equal to the reference voltage of the non-inverted input terminal of the operational amplifier IC3. As a result, the average value of the high-frequency current flowing in the detection resistor R6, that is, the load power which is the sum of power consumed by the preheating electrodes F1 and F2 of the discharge lamp LA is kept constant.
Main delay elements of the feedback circuit FB are the resistor R5 and the capacitor C8 of the integrating circuit IN, and the capacitor C2 of the error amplifier EA. The standard value of the delay time T due to those delay elements is expressed by T=(the resistance value of R5).times.(the capacitance value of the capacitor C8+the capacitance value of the capacitor C2). If this expression is applied to a conventional application example as shown in FIG. 12 in which the circuit constants are such that the resistor R5 is 9.1 k .OMEGA., the capacitor C8 is 100 nF, the capacitor C2 is 1.22 nF, and the delay time T is expressed by T=9.1 k .OMEGA..times.(100 nF+1.22 nF).apprxeq.900 .mu.s.
This delay time has been generally used taking such a case that excessive power is consumed by emission-less lighting of the discharge lamp, or the like, into consideration.
In the conventional discharge lamp lighting device, the feedback circuit FB keeps the load power in a constant value set by the reference voltage of the operational amplifier IC3, as described above. To change the load power, that is, to perform dim control for the discharge lamp LA, for example, such a method that the reference voltage of the operational amplifier IC3 is changed by changing the resistance value of the resistor R10 can be considered.
FIG. 14 is a graph showing a change of brightness X of the discharge lamp LA which is a fluorescent lamp, when the reference voltage V.sub.R of the operational amplifier IC3 is changed by changing the resistance value of the resistor R10 . In FIG. 14, the solid line designates the characteristic of a conventional example (the arrow shows a direction of the change of the reference voltage V.sub.R). In the conventional example, as the reference voltage V.sub.R of the operational amplifier IC3 gets lower, the frequency f becomes higher, and the brightness X of the discharge lamp LA gets darker. However, a jump phenomenon in which the brightness X of the discharge lamp LA changes discontinuously appears when the reference voltage V.sub.R takes a value V.sub.R1 or V.sub.R2. That is, when dim control is performed for a fluorescent lamp continuously in the conventional example, there arises a jump phenomenon in which the lamp gets dark suddenly at the point V.sub.R1 in the operation process to make the bright lamp dark, and the lamp gets bright suddenly at the point V.sub.R2 in the operation process to make the dark lamp bright. Therefore, there is a problem that such a jump phenomenon gives an unpleasant feeling, and particularly it appears conspicuously when the discharge lamp LA is a fluorescent lamp and the ambient temperature of the lamp is low.
On the other hand, the dotted line designates a desirable characteristic with no jump phenomenon. In addition, a change similar to that in the case where the feedback circuit FB is not operated is observed in FIG. 12 when the delay time is 900 .mu.s.
FIG. 15 is a graph showing a change, in enlargement, of electric characteristics with the passage of time in the fluorescent lamp LA at the reference voltage V.sub.R1 in FIG. 14, when the function of the feedback circuit FB is not actuated. In FIG. 15, AT designates a lamp current; VT, a voltage; and WT, electric power. The solid line shows the case of the conventional example, and the dotted line shows the case of an embodiment of the present invention, which will be described later and in which no jump phenomenon appears.
When the lamp current AT is reduced gradually so as to reduce the brightness of the fluorescent lamp, the lamp current AT begins to decrease suddenly at a point a so as to drop sharply to a point b. With this fact, the lamp power WT expressed by AT.times.VT.times.(power-factor) (substantially constant) is reduced suddenly in the same manner as the lamp current AT because the lamp voltage VT changes slowly. This change of the electric characteristics with the passage of time from the point a to the point b is about 1,000 .mu.s.
A change similar to that in the case where the feedback circuit FB is not operated is seen in FIG. 15 if the delay time is 900 .mu.s.
As has been described above, a jump phenomenon in which brightness of a fluorescent lamp changes suddenly is caused by a sudden change of the electric current or the electric power of the fluorescent lamp.
On the other hand, the delay time of the feedback circuit FB for keeping the load power constant in the above-mentioned conventional example is about 900 .mu.s. The value is close to the temporal change (1,000 .mu.s) of the electric characteristics at the jump time of the fluorescent lamp.
It is therefore difficult for the feedback circuit FB to effect the function to keep load power constant against a change of the load power, at the beginning of the jump time of the fluorescent lamp, which is an input of the feedback circuit FB. In addition, if the fluorescent lamp makes a jump once, the characteristic of the fluorescent lamp largely changes, so that, within a control range of the feedback circuit FB, the feedback circuit FB can not restore the characteristic to its original state before the jump.
The present invention has been achieved to solve the foregoing problems. It is therefore an object of the present invention to provide a discharge lamp lighting device in which dim control can be performed for a discharge lamp continuously and stably in a wide range, and which is simple in circuit configuration and low in price.